1. Field of the Invention
The present invention generally relates to implantable medical devices, and more particularly, pertains to telemetry for percutaneously transmitting analog and digital data between a programmer and an implantable medical device.
2. Description of the Prior Art
The earliest implantable medical devices were designed to operate in a single mode and with no direct percutaneous communication. Later it became clinically desirable to vary certain of the operating parameters and change modes of operation. This was accomplished through the use of programmers and other external devices which transferred commands percutaneously to the implanted medical device.
The communication between the implant and the external world was at first primarily indirect. The operation of an implantable cardiac pacer could be observed, for example, in the electrocardiogram of the patient. Soon it became known that data could be sent from the implanted cardiac pacer by modulating the stimulation pulses in some manner. This can only provide a low band pass channel, of course, without interfering with the clinical application of the device. Change of the pacing rate to indicate battery condition was a commonly used application of this technique.
As implantable cardiac pacers became more complex, the desirability to transfer more data at higher speeds resulted in the percutaneous transmission of data using a radio frequency carrier. The data to be transmitted is of two basic types, namely, analog and digital. The analog information can include, for example, battery voltage, intracardiac electrocardiogram, sensor signals, output amplitude, output energy, output current, and lead impedance. The digital information can include, for example, statistics on performance, time markers, current values of programmable parameters, implant data, and patient and unit identifiers.
The earliest RF telemetry systems transmitted analog and digital information in separate formats, resulting in inefficient utilization of the available power/bandwidth. Also, these modulation schemes tended to be less than satisfactory in terms of battery consumption, and do not lend themselves to simultaneous transmission of differing data types.
Many types of RF telemetry systems are known to be used in connection with implantable medical devices, such as cardiac pacemakers. An example of a pulse interval modulation telemetry system used for transmitting analog and digital data, individually and serially, from an implanted pacemaker to a remote programmer is disclosed in U.S. Pat. No. 4,556,063 issued to Thompson et al., herein incorporated by reference. An example of a modern pacemaker programmer for use with programmable cardiac pacemakers having RF telemetric capabilities is disclosed in U.S. Pat. No. 4,550,370 issued to Baker, herein incorporated by reference. A major difficulty in the use of such prior art systems is the need to maintain the communication link using continuous activation of the reed switch. This tends to place the radio frequency components in a less than optimal physical orientation. In addition to the bandwidth limitations found by the differing analog and digital formats, considerable bandwidth of digital channels tends to be consumed for forward error detection. Oftentimes, in previous systems this was accomplished by completely redundant separate transmissions.
A further problem of prior art RF telemetry systems involves the method of encoding and decoding digital data transmissions. Ordinarily, data is simply data value encoded and transferred as ordered digital data elements, with the byte (i.e. eight ordered binary bits) being the data element of choice. This tends to be a convenient data size in view of available hardware and software designs. A byte can be easily expressed as a two character hexadecimal number (i.e. in base 16) and/or converted to a decimal number as appropriate. Most often control commands and status reports are similarly data value encoded into hexadecimal bytes creating substantial redundancy in the data protocol and encoding/decoding hardware and software.